A thrust needle bearing, constituted of races, rollers (rolling elements) and a cage, is a bearing having various advantages such as high load capacity and high rigidity with a simple form. However, the number of the rollers is so large and internal clearances are so narrow that lubricating oil hardly spreads over rolling contact surfaces and spaces between the rollers and a pocket guide face of the cage. Further, the cage is formed by pressing a steel sheet in consideration of strength and cost in most cases. The inner surface (roller guide face) of a pocket hole formed by pressing is a shear plane having large surface roughness.
When the aforementioned bearing including the cage is used under a condition of lean lubrication or inferior lubricity, an oil film easily runs short between the rollers and the pocket guide face of the cage. Consequently, a roller 102 is worn on contact parts between the same and a pocket guide face of a cage 103, as shown in FIGS. 7 to 9. This wear deeply progresses on edge parts of roller 102, as shown in FIG. 9. Consequently, stress concentrates on the worn edge parts, to result in exfoliation of the roller 102 depending on load conditions. Further, surface-originating exfoliation is caused on a rolling contact surface of a race 101 coming into contact with the worn edge parts of the roller due to stress concentration and imperfect lubrication, as shown in FIG. 10.
In relation to the aforementioned problem of the roller worn on the contact parts between the same and the pocket guide face of the cage to result in exfoliation of the roller or the race and reduction of the life thereof, cages of resin (refer to Patent Document 1 and 2) are disclosed, while a cage partially made of resin at least in parts coming into contact with rollers is proposed in addition thereto (refer to Patent Document 3).
Patent Document 1: Japanese Patent Laying-Open No. 64-79419
Patent Document 2: Japanese Patent Laying-Open No. 4-357323
Patent Document 3: Japanese Patent Laying-Open No. 8-166014